1. Field of the Invention
This invention related to a method of dicing a workpiece of hard material, and more particularly to a method of mounting the workpiece in such a manner that chipping cracks and fins will be reduced during dicing. The invention also relates to a method of separating chips easily from an elastic fixing means after dicing.
2. Description of the Related Art
Conventionally, in the semiconductor industry, dicing takes place to cut chips off a wafer on which a large number of integrated circuits are formed. The conventional dicing method will now be described with reference to FIGS. 7 through 9 of the accompanying drawings.
FIG. 7 is a schematic vertical cross-sectional view of a wafer mounted ready to be diced. The mounted wafer 2 comprises a wafer mount frame 3, and a resin adhesive tape 4 for fixing the wafer 2 to the wafer mount frame 3. The wafer mount frame 3 has a large opening larger in diameter than the wafer 2. The resin adhesive tape 4 includes a substrate 4a of vinyl chloride, and an adhesive layer 4b using an acrylic adhesive agent.
The main procedure of mounting the wafer 2 will now be described with reference to FIG. 8.
For mounting the wafer 2, firstly the resin adhesive tape 4 is stuck to one surface of the wafer mount frame 3, having such a size as to back the opening of the wafer mount frame 3. Then the excessive part of resin adhesive tape 4 except the part backing the opening of the wafer mount frame 3 is cut off. After the wafer 2 is placed on a mount table 5 heated at 80.degree. C., the wafer mount frame 3 with the resin-adhesive-tape side facing up is placed on the mount table 5 in such a manner that the wafer 2 is received in the opening of the wafer frame 3. As shown in FIG. 7, the wafer 2 is then fixed to the resin adhesive tape 4 using a rubber roller 6, finalizing the mounting of the wafer 2.
The procedure of cutting the wafer 2 into chips is described as follows.
FIG. 9 is a diagram schematically showing the wafer 2. On the wafer 2, chips 10 are orderly arranged in rows and columns, there being a groove, called a scribe line 11, between adjacent chips. The wafer 2 is cut into the individual chips along the scribe lines 11 using a dicing saw. At that time, as shown in FIG. 9, the wafer 2 is cut in the direction of rows (CH2) and then in the direction of columns (CH1). The extent (depth) of this cutting of the dicing saw is usually 30 to 40 .mu.m.
Thus, the mounted wafer 2 is cut into chips by the dicing saw. The chips stuck to the resin adhesive tape 4 are separated from the adhesive tape 4 by ultrasonic washing.
According to the conventional dicing method, however, stresses due to the dicing saw are exerted directly on the fixed wafer, which would cause chipping cracks in the cut surface of the wafer partly because the adhesion of the resin adhesive tape to fix the wafer is too strong and partly because the resin adhesive tape can escape to only a limited extent to absorb the stresses during the dicing. Since silver, which is used as a metal of the rear surface (cathode surface) of the wafer is malleable and ductile and hence pulls the semiconductor device while the wafer is being cut, it would facilitate not only the occurrence of chipping cracks but also the development of fins on the rear surface. Particularly during the cutting in the direction of columns CH1, since it takes place after the cutting in the direction of rows, stresses would be absorbed by the malleability of the resin adhesive tape so that chipping cracks will hardly occur in the wafer. On the contrary, during the cutting in the direction of rows CH2, since the wafer is fixed too firmly by the resin adhesive tape, chipping cracks are apt to occur.